1. Field of the Invention
The present invention relates to a cascode amplifier primarily for use in mobile communication devices such as cellular phones.
2. Background Art
Increasing effort is currently being spent in developing cascode amplifiers formed by a CMOS process which are used as a means for reducing the cost of power amplifiers for CDMA cellular phones and other types of cellular phones (see, e.g., Japanese Laid-Open Patent Publication No. H05-259765).
FIG. 16 is a circuit diagram showing the basic configuration of a cascode amplifier. The cascode amplifier is shown within a dashed line rectangle, and the other circuit components shown are required for forming a power amplifier. Transistors Tr1 and Tr2 are n-channel MOS transistors and are connected together in a cascode configuration. An amplifier using cascode-connected transistors is referred to as a “cascode amplifier.”
The gate of the transistor Tr1 is connected to an RF input signal terminal IN through an input matching circuit and also connected to a gate bias terminal Vg1. The source of the transistor Tr1 is grounded. That is, the transistor Tr1 is a source-grounded transistor.
The gate of the transistor Tr2 is connected to a gate bias terminal Vg2, and grounded through a capacitance C1. That is, the transistor Tr2 is a gate-grounded transistor. The source of the transistor Tr2 is connected to the drain of the transistor Tr1. The drain of the transistor Tr2 is connected through a transmission line L1 to a drain power terminal Vd of this cascode amplifier, and also connected through an output matching circuit to an RF output signal terminal OUT. The transmission line L1 has a specific electrical length and acts as an inductor.